The shape of pebbles revealed the existence of ancient Martian rivers.

A recent study published by Nature Communications uses the images taken by NASA's Curiosity rover to estimate the length of ancient Martian rivers. According to the study, these rivers extended beyond 30 miles up to the rim of Gale crater. This suggests that 3 billions years ago a large and sustained river system may have existed on Mars. The length of the ancient rivers was estimated in two steps by using Curiosity's images of rounded pebbles. In the first step the original mass (at the time of fragmentation) of those pebbles was estimated using solely the current contours. In the second step, based on the mass loss, the travel distance in the riverbed was estimated. The first step is primarily geometrical and it is based on the mathematical model describing the evolution of pebble shapes under abrasion. This model predicts that in this process the average number of static balance points is reduced thus pebbles evolve towards the Gömböc shape. This prediction was compared to data collected in field study and laboratory experiments. This led to the verification of the equations which then could be applied to back-track the shape evolution of Martian pebbles. The result was broadly reported in the media: following the press release by the University of Pennsylvania, among others the Los Angeles Times (which also featured the article on its cover page), the Australian, the Irish Examiner and the Daily Mirror as well as the science portals space.com and phys.org published articles.